This invention relates to a rubber material vulcanized with a sulfur vulcanization system, which has excellent oil resistance and antidegrading properties and is particularly adapted for use in contact with casing head gas and/or crude oil.
Today, more than 75% of world energy supply relies on crude oils and natural gases recovered from wells. In spite of the estimated future increase of energy consumption, however, it has become difficult to discover new oil or gas wells. Efforts have therefore been made to discover new wells, and attempts have been made at the same time to increase the yield of crude oils or natural gases from the wells now in operation.
To recover crude oils or natural gases, drilling of wells is first performed. To increase the drilling efficiency at this time, various improved drilling oils, drilling fluids, etc. are used. Then, an iron pipe called a casing pipe is inserted from above the ground into a bare hole with a diameter of 20 cm to about 60 cm formed in the stratum by drilling. The well is completed by using cement, rubber materials, etc. so as to enable oil recovery. Crude oil is recovered from the completed well by spontaneous flowing, pumping, etc.
However, rocks, mud and sand from the drilled stratum, and the drilling oil or fluid used during the drilling operation deposit and build up near the well, and this may markedly hamper recovery of crude oil. In such a case, measures are taken to increase oil producing ability by, for example, adding a liquid obtained by mixing kerosene, light oil or diesel oil with an acid, an alkali, chlorine or various surface-active agents to the well in advance to dissolve or disperse the sediment, etc. and discharging them out of the well.
When the crude oil has difficulty of flowing into the well because of asphalt or wax components deposited near the well as a result of long-term operation, or because of the high viscosity of the crude oil in the oil reservoir, it is the practice to force steam into the well to dissolve the deposits or decrease the viscosity of the crude oil.
The ratio of recovery of crude oil from the oil reservoir is very low if the flow of the crude oil into the well is effected only by natural energy. Thus, in order to increase the yield of oil, various treatments have been performed. For example, water is forced into the oil reservoir to increase the pressure of the oil reservoir, or steam is added to increase the temperature of the oil reservoir. Or CO.sub.2 or LPG is forced into the crude oil to reduce its viscosity, or the surface tension between oil and another material is reduced. As a result, the inside of the well has very high pressures and temperatures (usually at least 150.degree. C.). Crude oils or natural gases recovered from the wells contain large amounts of corrosive CO.sub.2 and sulfur compounds such as H.sub.2 S.
Accordingly, an environment with which rubber materials used for well completion make contact is very severe.
An acrylonitrile/butadiene copolymer rubber (to be referred to as NBR) has previously been used in large amounts as rubber materials for wells (oil and gas wells). Rubber materials composed of NBR, however, become useless within a short period of time under severe high-temperature, high-pressure environmental conditions by the action of crude oil, hydrocarbon gases, H.sub.2 S, CO.sub.2, acids, alkalies, etc. Fluoro-rubber proposed as a material intended for removing the defect of NBR has low mechanical properties such as tensile strength and tear strength, and does not have sufficient resistance to the action of the aforesaid substances. For this reason, it has been desired to develop new rubber materials.
Rubber materials used in wells are required not only to have high mechanical strength and resistance to crude oils, but also have resistance to additive materials used in various treatments performed to increase the yield of oil, H.sub.2 S, CO.sub.2, or a mixture of these, and resistance to heat. If they lack a balance of mechanical strength, resistance to the aforesaid substances and heat resistance, they do not function as rubber materials. Ideally, such rubber materials can be used until the well dries up. In practice, however, they have to be exchanged every time they become degraded. Since this requires extra work for exchanging and for interruption of production owing to exchanging, it gives rise to an economic problem.